For dart frogs, the more common worry is cooling rather than heating.
Darts prefer temps in the low 70s with drops into the high 60s at night.

You could use a heat mat, but I have heard that it can crack moist glass, which, in a dart viv, can be a problem
you could use a heat bulb or a ceramic heat emitter but then comes the humidity task and also the potential overheating.

The best thing to do in my opinion is to build your tank and monitor the temps. If you need to get something to keep temps up you will be able to do it before any frogs are put in